This application is related to commonly owned patent applications: U.S. Ser. No. 08/236,444, entitled "Power Control and Delivery in Plasma Processing Equipment" by Roger Patrick and Frank Bose, filed May 2, 1994; U.S. Ser. No. 08/106,017, now U.S. Pat. No. 5,407,524, entitled "End-Point Detection in Plasma Etching by Monitoring Radio Frequency Matching Network" by Roger Patrick and Frank Bose, filed Aug. 13, 1993; and U.S. Ser. No. 08/027,995, now U.S. Pat. No. 5,401,350, entitled "Coil Configurations for Improved Uniformity in Inductively Coupled Plasma Systems" by Roger Patrick, Frank Bose, Philippe Schoenborn and Harry Toda, filed Mar. 8, 1993; all of the aforementioned applications assigned to LSI Logic Corporation.
1. Field of the Invention
The present invention relates to plasma processing systems and, more particularly, to a method and apparatus for dynamically controlling the delivery of radio frequency power in plasma process systems.
2. Description of the Related Technology
Ionized gas or "plasma" may be used during processing and fabrication of semiconductor devices, flat panel displays and in other industries requiring etching or deposition of materials. Plasma may be used to etch or remove material from semiconductor integrated circuit wafers, sputter or deposit material onto a semiconducting, conducting or insulating surface. Creating a plasma for use in manufacturing or fabrication processes, typically, is done by introducing a low pressure process gas into a process vessel chamber surrounding a work piece such as an integrated circuit wafer. The molecules of the low pressure gas in the chamber are ionized into a plasma by the radio frequency energy (power) source after entering the chamber, and the highly reactive plasma flows over the work piece. The process vessel is used to maintain the low pressures required for the plasma and to serve as a structure for attachment of one or more radio frequency energy sources.
Plasma may be created from a low pressure process gas by inducing an electron flow which ionizes individual gas molecules by the transfer of kinetic energy through individual electron-gas molecule collisions. Typically, electrons are accelerated in an electric field such as one produced by radio frequency ("RF") energy. This RF energy may be low frequencies (below 550 KHz), high frequencies (13.56 MHz), or microwaves (2.45 GHz). A plasma etching system consists of a radio frequency energy source and a pair of electrodes. A plasma is generated between the electrodes while the work piece, such as a semiconductor wafer, is planar with one of the electrodes. The chemical species in the plasma are determined by the source gas(es) used.
Plasma etching methods and apparatus are generally illustrated in U.S. Pat. Nos. Re 30,505 and 4,383,885. These patents illustrate plasma etching systems. A method and apparatus for obtaining a substantially parallel (planar) plasma for processing of integrated circuit wafers is described in U.S. Pat. No. 4,948,458. A typical plasma etching system may consist of an enclosure having an interior bounded at least in part by a radio frequency transparent window. A planar coil is disposed proximate to the window, and a radio frequency energy source is coupled through an impedance matching circuit to the coil. The planar coil radiates the radio frequency energy such that a planar magnetic field is induced in the interior of the enclosure. A plasma is generated thereby from the process gas. This plasma reacts with the surface of the semiconductor wafer, etching it away.
Plasma may also be used in chemical vapor deposition (CVD) to form thin films of metals, semiconductor or insulator materials onto a work piece such as a semiconductor wafer. Plasma-enhanced CVD uses the plasma to supply the required reaction energy for deposition of the desired materials. Typically, radio frequency energy is used to produce this plasma.
Control and delivery of the power in a plasma discharge is of fundamental importance in plasma processing, including etching, sputtering and deposition systems. Uniformity and repeatability are critical aspects of plasma etching. Uniformity of the plasma is required in order to uniformly remove a desired layer from a semiconductor wafer while minimizing the undesired etching of an underlaying layer. Repeatability of the plasma etching process allows for increased manufacturing yields and a wider latitude in the manufacturing process tolerances.
The most commonly used method of obtaining a predetermined radio frequency power is to use a matching network between a radio frequency power source and the plasma discharge chamber electrode or coupling coil. The matching network transforms the impedance (capacitive reactance) of the plasma discharge into a substantially resistive load for the radio frequency power source. The power source is then set to a predetermined power level dependent upon the process parameters desired.
In present plasma systems, radio frequency power is monitored and controlled at the generator output on the assumption that the power losses in the matching network are negligible. However, radio frequency power delivered to the plasma chamber has been found to be substantially less than the generator power because of losses in the matching network and other associated components of the radio frequency power generation system. The amount of actual power in the plasma chamber greatly affects the process conditions. Significant variance in actual power delivered to the plasma chamber may unexpectedly change the anticipated contribution of other process variable parameters, such as pressure and etch rate.
For better control of the plasma process and a more reliable and repeatable deterministic insight into the actual physical effects of process parameter changes, it is preferable to control the characteristics of the radio frequency power actually delivered to the process chamber. A system and method for determining and controlling the radio frequency power parameters being delivered to the plasma chamber is more fully described in co-pending and commonly owned patent application, U.S. Ser. No. 08/236,444, entitled "Power Control and Delivery in Plasma Processing Equipment" by Roger Patrick and Frank Bose, filed May 2, 1994, and incorporated herein by reference.
Plasma processes are characterized and ultimately optimized by evaluating the process results on a work piece such as, for example, a semiconductor wafer. The process results may be characterized as a function of the plasma process parameters, e.g., radio frequency power, plasma gas pressure, plasma chamber electrode spacing, types of gases uses to form the plasma, and the gas flow rates. Use of Response Surface Modeling (RSM) may be employed not only to find the optimum process performance but also to find the most stable plasma etching regions in order to reduce noise errors in the process.
Referring to FIG. 1, the variation in etch rate over time for a plurality of semiconductor device wafers is graphically illustrated. In the graph of FIG. 1, the dots on the horizontal axis represent the plurality of wafers being etched and the vertical axis represents the respective etch rate for each of the plurality of wafers. A significant spread in the etch rate of the wafers can be noted.
Even with the most careful process optimization procedures this etch rate spread is significant because present art plasma etching tools such as the LAM TCP 9400 manufactured by Lam Research Corporation, Fremont, Calif., utilizes static control of the power to the plasma chamber. However, the plasma etching process is a dynamic process, where most of the dynamics are in the first few seconds of the gas discharge, when the plasma stabilizes.
Improved control of the radio frequency power at the plasma chamber will help reduce the range of the etch rate distribution. A system for control of the radio frequency power at the plasma chamber is more fully described in P. Rummel, "Monitoring and Control of RF Parameters Near Plasma Loads," Industrial Heating, May 1991. In addition, the aforementioned patent application, U.S. Ser. No. 08/236,444, entitled "Power Control and Delivery in Plasma Processing Equipment" more fully illustrates power control and delivery at the plasma chamber.
Measuring and controlling the radio frequency power at the plasma chamber electrodes, however, does not eliminate all of the non-uniformities and the wide variance (spread) in the etch rates between semiconductor wafers sequentially processed in the plasma chamber. It has been determined that the remaining non-uniformities and the spread in the etch rates are caused substantially by the way the plasma gas discharge is started. The plasma discharge is unstable during the first few seconds of the gas becoming a plasma. The plasma instability during these first few seconds causes more of the etch non-uniformities and spread in the etch rates than does the bulk etch time when the plasma is stable.
FIGS. 3A and 3B illustrate the power taken by the plasma load as a function of time for a plurality of wafers, e.g., W7, W8, W11, W13, W15, W17, W19 and W23 (FIG. 3C). The load power versus time required during the start of the processing for each wafer is different (FIG. 3B is an expanded scale of FIG. 3A). It is this difference that has such a significant effect on the overall uniformity (or lack thereof) of the etch process for each wafer.
What is needed is a way to optimize the plasma process during the first few seconds of creating a plasma from the gas by the application of the radio frequency power. It is therefore an object of the present invention to dynamically optimize the etching process during the formation (start) of the plasma.